This invention relates to improved methods and apparatus for noninvasively monitoring dynamic cardiac performance of a living subject.
Cardiac output is an important parameter of the circulatory system since it is a direct measure of systemic blood flow and thereby the transport of oxygen and nutrients to all tissues of the body. Heart disease can result in a decrease of cardiac output leading to inadequate nutrition of the cells of the body. Therefore, measuring cardiac output is useful in monitoring the critically ill patient, in rehabilitation medicine and in medical screening procedures.
The most common methods of measuring blood flow used at this time are invasive and are therefore not suitable for many patients and circumstances. A simple to use, noninvasive apparatus to measure or accurately estimate cardiac output could be applied to all patients.
Cardiac output may be expressed as the product of heart rate and volume of blood pumped per beat of the heart. Thus, under conditions of a consistent heart rate and stroke volume, ##EQU1##
Cardiac output or blood flow is also directly proportional to mean blood pressure and is inversely proportional to the peripheral resistance of the arterial system through which the blood flows (i.e. the aorta).
Because of the importance of changes in cardiac output and the difficulties in its direct measurement, the estimation of cardiac output and stroke volume from blood pressure pulse waveforms has been extensively studied (see "Engineering Hemodynamics: Applications to Cardiac Assist Devices" by Walter Welkowitz, first published by D. C. Heath & Co., First Ed. 1977; Second Ed. NYU Press, 1987).
In one approach, McDonald (1974) recorded two pressure pulses 3-5 cm apart within the ascending aorta (an invasive technique). Both pulses were subjected to Fourier analysis and the apparent phase velocity was calculated for each harmonic of the pulses. The phase velocities were applied to the Wormsley equation to calculate aortic flow and stroke volume (the integral of aortic flow throughout one cycle). A problem encountered with this method is that the aorta exhibits non-uniform geometric and elastic properties and thereby renders the Wormsley calculation inaccurate. To overcome that problem, Muthukrishnan and Jaron (1975) used a parameter optimization technique to compute aortic input impedance in a manner similar to an earlier proposal of Strano, Welkowitz and Fich (1972) based upon an aorta model developed still earlier by Welkowitz and Fich (1967). Instantaneous aorta flow waveforms were calculated by Muthukrishnan from the input impedance and the proximal aortic pressure. The aortic flow waveform estimated by this analysis closely matched an actual flow waveform measured using a electromagnetic flow meter. The Muthukrishnan method, however, which is based upon two internal pressure measurements, is an invasive stroke volume technique.
Min, Welkowitz and Kostis (in 1978) described an extension of this technique using two simultaneous noninvasive pulse contour measurements acquired with two piezoresistive pulse transducers combined with an ultrasonic measurement of aortic diameter (see Proc. 6th New England Bioengineering Conference. pp. 15-19). In addition to the foregoing pressure and pulse techniques, there has been a great deal of work in the areas of doppler ultrasound and bioimpedance measurements (see U.S. Pat. No. 4,562,843--Djordjevich et al.; Welkowitz et al., "Biomedical Instruments: Theory and Design", Academic Press, New York, 1976; Kubicek et al., "Impedance Cardiography as a Non-invasive Method of Monitoring Cardiac Function and other Parameters of the Cardiovascular System", Ann. N.Y. Acad. Sci., 170: 724, 1970).
It is known that hemodynamic characteristics of the aorta can be simulated by an R-L-C electrical linear network. Researchers have developed various aorta simulation models to perform estimations and calculations for different purposes. Based upon an equivalent electrical circuit model developed by Watts (1974), an aortic flow waveform has been calculated from a carotid pulse waveform. A corresponding cardiac output then may be computed. A microcomputer can be used for this simulation and calculation.
The knowledge that the aorta can be represented by an electrical circuit model has led some researchers to seek a noninvasive method and apparatus for cardiac output monitoring using circuit simulation. One such modeling technique is described in our above-referenced Application Ser. No. 07/502,409, the disclosure of which is herein incorporated by reference.